The invention relates to an installation for coating objects with a powder, in particular for powder painting, having
a) a booth, in which powder can be applied to the objects by at least one application device;
b) a recovery device for the excess powder, which does not adhere to the objects during coating;
c) a storage container for fresh powder;
d) a mixing container, which can be supplied with excess powder from the recovery device and fresh powder from the storage container;
e) a screening machine, which can be supplied with mixed powder via an outlet valve of the mixing container;
f) an application container, in which screened mixed powder can be temporarily stored for removal by the application device, and
g) a weighing device, which measures the weight of the application container.
It is known that when coating objects with powder there is a considerable amount of excess powder (so-called xe2x80x9coversprayxe2x80x9d), which does not adhere to the objects to be coated and for cost reasons must be recovered. This recovered excess powder is mixed in a particular ratio with fresh powder in a mixing container, after which it is screened in a screening machine and generally supplied to an application container, from which the application device then removes the powder which it has dispensed.
In an installation of the type mentioned at the outset which is known from the market, the application container is situated very close to the application device and is connected to the screening machine via a relatively long line. The application container is subjected to relatively low-frequency vibrations in order to keep the mixed powder situated therein flowable even in the vicinity of the walls. The weight of the application device is monitored with the aid of a weighing device and the supply of mixed powder from the screening machine is initiated when the filling weight of the application container falls below a particular value. This weighing device operates sluggishly for two reasons: on the one hand, the electronics have to be provided with a time constant which is long compared with the period of the vibrations to which the application container is subjected; on the other hand, the refilling of the application container takes a relatively long time owing to the long connection path to the screening machine and cannot be performed reliably and completely within the relatively short time lying between two coating operations. For this reason, it is not possible either to use the weight or change in weight of the application container, determined by the weighing device, as a reliable measure of the quantity of powder instantaneously dispensed per unit of time.
The object of the present invention is to design an installation of the type mentioned at the outset in such a way that the quantity of powder dispensed by the application device for each object to be painted can be unambiguously measured and documented, continuously in real time.
This object is achieved according to the invention in that the screening machine
h) is arranged in the immediate vicinity of the mixing container;
i) serves at the same time as the application container;
k) is not subjected to any low-frequency vibrations;
l) is provided with a controllable outlet valve.
The invention is based on the realisation that, in order to utilise the output signal of the weighing device as a real-time measure of the instantaneously dispensed quantity of powder, various conditions must be fulfilled: firstly, the conveying path between the outlet valve, which controls the charging of the application container, and the application container itself, whose weight is to be monitored, must not be too long, as this would make control of the refilling operation overall too sluggish. Consequently, the present invention uses the screening machine, which is arranged directly by the mixing container, at the same time as the application container. In this way, the time which the mixed powder takes to travel from the mixing container to the application container is shortened to a negligible amount and at the same time the outlay on apparatus is reduced. Secondly, the screening machine must be provided with a controllable outlet valve, in order to allow a defined quantity of mixed powder, sufficient for the complete coating of at least one object, to be weighed into the screening machine at the start of the coating operation. Thirdly, the screening machine must not vibrate at such a low frequency that, in order to process the electrical weighing signal, time constants are required which are comparable with the times which the mixed powder takes to cover the distance to the application device., in particular on no account comparable with the cycle times of the coating operation in the corresponding booth.
If these conditions are fulfilled, it is possible to correlate the change of the screening machine in time directly with the quantity of powder removed per unit of time. Since these measurement results are obtained in real time, it is continuously known which object has had the measured quantity of mixed powder applied to it. Faults in the coating operation, which are determined directly during this operation or else in the subsequent evaluation of stored data, can be unambiguously assigned to a particular object, which can then be segregated as being defectively coated, and optionally refinished.
It is particularly advantageous for the screening machine to be arranged directly beneath the mixing container and the mixing container to have a discharge pipe, via which, after opening the outlet valve of the mixing container, the mixed powder passes in free fall into the screening machine. In this way, the charging of the screening machine is accomplished in a particularly trouble-free and quick manner, so that it is possible, even in the case of rapid cycle times of the coating operation, for the screening machine serving as the application container to be filled between two coating operations. In addition, given that the screening machine is charged in free fall, no secondary air and no additional energy are introduced into the screening machine, which, as experience shows, would otherwise lead to deposits on the screening material.